The invention relates to a tube bundle heat exchanger for controlling a wide performance range.
For the purpose of cooling medium flows, in particular of gases, in numerous process technology plants such as gasification plants, thermal and catalytic cracking plants, steam reforming plants etc., as a rule heat exchangers, in particular tube bundle heat exchangers (radiators), are used, in which the medium flows to be cooled flow through straight heating surface tubes and in this process transfer the existing heat of the hot medium flow by way of the tube wall to the cooling medium that surrounds the tubes.
It is the main task of such a heat exchanger or tube bundle heat exchanger, as stated above, to transfer heat between two media, wherein a particular quantity of heat is removed from the one medium (hot medium), and an adequate quantity of heat is fed to the other medium (cooling medium). It is well known that the quantity of the heat transferred depends on the size of the heat exchanger, on the heat transfer coefficients of the two media, and on the temperature difference between the two media. In single-phase media the medium temperature changes with the infeed of heat or with the removal of heat. In this case the temperature gradient over the device length of the heat exchanger resembles an exponential function.
As a rule, a tube bundle heat exchanger comprises a large number of heating surface tubes, a pressure shell that surrounds the heating surface tubes and that forms a shell side, and two tubesheets, between which the heating surface tubes are arranged. The first medium flows through the tube inlet chamber of the heat exchanger, then through the heating surface tubes and through the tube outlet chamber of the heat exchanger. The second medium flows by way of a nozzle into the shell side of the heat exchanger, flows several times around the individual heating surface tubes, and subsequently flows through a second nozzle out of the heat exchanger.
The two media can flow in a heat exchanger or tube bundle heat exchanger in the same axial direction of the heat exchanger (co-current flow), or one of the two media can flow within the heat exchanger in the opposite direction relative to the other medium (counter current flow). The temperature gradient of the heat exchange of the media with counter current flow differs from that with co-current flow and thus results in a difference in the average logarithmic temperature difference between the two media. The quantity of heat transferred between the two media is thus different in both circuit connections, i.e. counter current flow connection or co-current flow connection.
The output of the heat exchanger or of the tube bundle heat exchanger can change as a result of fouling (build-up of deposits or dirt within the heating surface tubes) or as a result of other influences over the service life of the tube bundle heat exchanger; a situation which results in the need for control intervention. At the same time there is often the need to adjust the quantity of heat to be transferred or the exit temperatures of the medium to the desired operating load. Frequently, bypass control comprising a bypass pipe and a three-way mixing valve, i.e. a regulated three-way valve, is used for controlling the exit temperatures of the medium and thus the thermal output of the tube bundle heat exchanger. In this process, part of the medium flow is removed from the main flow before it is fed to the tube bundle heat exchanger, and is made to bypass the tube bundle heat exchanger. The reduced flow quantity of a medium reduces heat transfer and, by way of the changed exit temperature of the medium, influences the average logarithmic temperature difference. The control range or the control intervention that is achievable with this bypass arrangement is, however, relatively modest.
It is the object of the present invention to create a tube bundle heat exchanger with a bypass system in which the above-mentioned disadvantages are avoided, or in which the exit temperatures of the media and the quantity of heat to be transferred are controllable in a very wide range.